I've done a whole Bren body in this same stuff but I got it from here
Frost Metal Blacking Kit and Solutions | Frost Restoration
I think its selenium based and has had bad reports of it just rubbing off but I did the Bren about 6 years ago and it still looks 100%, no rubbing off and I did try to scrape it to test and it didn't scratch. I think the key to this stuff is to follow the instructions to the letter and prepare well (I used my shot blast cabinet and lots of disposable gloves and made sure the metal was warm).